The diameters of HiPco single-wall carbon nanotubes were increased by heat treatments. The original diameters estimated from Raman spectra were In the range of 0.79 to 1.2 nm; they Increased with the heat-treatment temperature, reaching about 1.47 nm after 1800 degreesC heat treatment. Thermogravimetric analyses showed that the burning temperature of the HiPco tubes rose by about 350 degreesC after 1800 degreesC heat treatment, and the metal content, which was about 30% in the original HiPco, decreased to about 2% after heat treatment at 1800 degreesC.